A deadly Pennsylvania explosion that left three dead, including a 2-year-old boy, over the weekend has been ruled a murder-suicide, officials said Thursday.

In a series of letters mailed to the Allentown Police Department before his death, 26-year-old Jacob Schmoyer indicated his intentions to purposefully detonate the homemade car bomb that killed him, his young son, J.J., and 66-year-old David Hallman, whom police described as an acquaintance of Schmoyer's.

But what led the father to kill his own son and Hallman remains a mystery.

Meanwhile, Schmoyer's sister, Tina, said she can't believe he was behind the blast.

"I was there; I lived with him," she told Inside Edition. "There was nothing, absolutely nothing, to go with this."

But authorities said letters sent by Schmoyer paint a different picture.

"He was miserable. Basically, the four letters describe a miserable life. He was unhappy himself," Don Robinson, the special agent in charge at the ATF Philadelphia, said in a press conference Thursday.

"There was a lot of hatred there, and obviously, some directed at Mr. Hallman and his son," he added.

One report suggested that J.J. had autism, a sore point for Schmoyer, whose letters apparently indicated he had something against it.

As for Hallman's family, they want to know why he was targeted, but they say he would be happy that his beloved Jack Russell terrier Skippy, who was nearby, survived the blast.

"Skippy was the love of his life," said Hallman's niece, Christine Erdman. "He loved her."